Objectives are: 1) the evaluation of the effect of platelet transfusion in preventing hemorrhage in thrombocytopenic children with acute leukemia, 2) detemination of the value of HL-A typing in selection of platelet donors, and 3) evaluation of the need for platelet transfusions in afebrile leukemia patients with thrombocytopenia. Two prospective randomized studies will be conducted to answer the above questions concerning optional platelet transfusion prophylaxis and therapy.